Scattering occurs when parallel streams of photons (light waves) encounter an obstacle and change direction.
In this context, scattering refers to various forms of deviation linked to diffusion, dispersion, interference, reflection, and refraction, as well as the composition and surface properties of different materials.
Scattering does not take place:
- When parallel rays of light reflect off a smooth, flat surface like a mirror, producing a distortion-free reflection.
- When parallel rays of light reflect off a smooth convex surface (although the reflection appears magnified).
- When parallel rays of light reflect off a smooth concave surface (although the reflection typically appears smaller and inverted).
- When parallel rays of light pass through translucent materials containing dissolved substances like dyes.
About regular scattering
- Regular scattering of light refers to the predictable deviation and deflection of light rays when they pass through or reflect off smooth and transparent surfaces.
- Regular scattering takes place when:
- Parallel rays of light reflect off a curved surface like a convex or concave mirror. In this case, regular scattering creates a magnified or reduced distortion-free reflection.
- Parallel rays of incident light are deflected by objects like raindrops and prisms that have smooth surfaces and transparent interiors.
- Regular scattering is frequently accompanied by chromatic dispersion, which separates white light into its constituent spectral colours.
- Chromatic dispersion refers to the way that light, under certain conditions, separates into its component wavelengths and the colours corresponding with each wavelength produce a rainbow effect.
- On a microscopic level, all types of scattering follow the laws of reflection and refraction (Snell’s law).
Let’s consider two cases of regular scattering in more detail:
- When parallel rays of light with a single wavelength strike and enter an object like a raindrop or prism, their path depends on the initial point of impact, the refractive indices of air and water, and the object’s surface properties.
- When parallel rays of incident light with a single wavelength meet the curved surface of a transparent medium at various points, the different angles at which they strike the surface and experience deflection mainly determine how they scatter as they exit the medium.
About random scattering
- Random scattering refers to the scattering of light rays in various directions when they encounter irregularities or imperfections on a surface.
- Random scattering takes place when a material reflects or transmits light rays in multiple directions.
- Random scattering can produce reflections that appear soft, lack distinct detail, or even result in no reflection at all.
- When light passes through sheets of glass with irregular yet smooth surfaces, it distorts the view of the world beyond. Random scattering is responsible wherever the image appears blurry and confused.
- A reflection that is free of the effects of random scattering is called a specular reflection. Mirrors generally produce specular reflections.
- Diffuse light involves the random scattering of light in all directions when it encounters a rough or uneven surface.
- Diffuse light is produced when it bounces off rough or uneven surfaces, scattering light in every direction.
- Diffuse light can be the result of the overall structure and composition of the medium, such as when light is transmitted through the interior of a medium that:
- Contains foreign material
- Contains suspended particles of different sizes
- Has an irregular interior structure
- Has variations in density
- Absorbs light and then re-emits it
- Translucent materials containing dissolved substances such as dyes don’t cause random scattering.
- On a microscopic scale, all objects adhere to the law of reflection; however, when surface irregularities are larger than the wavelength of light, the light undergoes scattering leading to diffusion.
About scattering in raindrops
- Regular scattering of light in raindrops results from both refraction and chromatic dispersion.
- Regular scattering occurs when light is scattered by particles that are much smaller than the wavelength of the radiation.
- Refraction occurs when light changes speed and direction as it passes from one transparent medium to another.
- Chromatic dispersion is the phenomenon where light separates into its various colours and becomes visible to a human observer.
- Scattering in raindrops obeys the laws of both reflection and refraction, commonly referred to as Snell’s law.
Here are three related descriptions of what causes scattering when visible light strikes a raindrop:
- When light of a specific wavelength strikes the surface and enters a raindrop its subsequent path depends upon the point of impact, and the refractive indices of water and air.
- When rays of light of a single wavelength strike a raindrop at different points, scattering is primarily determined by the angles at which they enter the droplet.
- The interaction between refraction and chromatic dispersion gives rise to the appearance of rainbow colours when parallel white light rays strike various points on the surface of a raindrop.